Abstract

Reliable estimation of formation tectonic stresses plays an important role in geomechanics and managing well-planning problems, where detailed knowledge of stress magnitudes and directions is needed for accurate prediction of wellbore stability, characterization of reservoir behavior, anisotropy analysis, etc. One of the prevalent methodologies for stress evaluation involves cross-dipole measurements followed by dispersion analysis at sonic frequencies. The crossover of the dipole dispersion curves indicates the presence of differential stresses around the wellbore. Fast and slow shear slownesses are split at low frequencies and determined by the far-field stresses. On the other hand, it is known that the depth of investigation at sonic frequencies is relatively far from where the largest variation of the stress-induced anisotropic formation slownesses exists which is near wellbore area.

As an alternative approach, this paper presents the importance of the ultrasonic azimuthal slowness measurements (microsonic) for characterizing stress effects in boreholes. The abilities of such measurements are numerically evaluated by simulating elastic waves propagating in formations subjected to triaxial stresses. Taking advantage of the short wavelength, the ultrasonic measurement is found to be highly sensitive to formation properties at shallow distances from the borehole wall and suitable for mapping stress-induced variations of compressional and shear slownesses around the wellbore. These slownesses are evaluated from simulated waveforms as a function of the azimuth around the borehole using a semblance processing algorithm.

Introduction

Knowledge of tectonic stresses is required for key oilfield operations including well planning, drilling, wellbore stability analysis and enhanced production. One of the prevalent methods for characterizing tectonic stresses is to use borehole acoustic logging tools, such as wireline or Logging-While-Drilling (LWD) sonic tools. It is known that even in homogeneous and isotropic formations, the presence of a borehole disturbs the stress fields in the vicinity of the wellbore, alters elastic properties of surrounding formations, and, therefore, results in stress-induced acoustic anisotropy.

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